1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method and particularly to a technology for obtaining a stable image processing result for a breast image with a large personal difference in mammography for breast diagnosis using an image of a breast photographed by X-ray.
2. Description of the Related Art
As a diagnosis using a medical image obtained by photographing a subject, mammography (breast X-ray photography) is conventionally known, for example. Breasts are different in size or mammary gland quantity among people and are said to be an organ with the largest personal difference in human tissues.
Thus, features of the breast images taken by the mammography have large personal differences, and density/contrast of the image becomes largely different depending on a ratio between mammary gland tissues and fat tissues, distribution state of the mammary gland tissues and the like. For example, an image of a breast with a large mammary gland amount often seen in a young generation can easily become a blurred image with a low contrast as compared with a breast with a small mammary gland amount.
In a breast, the mammary gland tissues and the fat tissues are present in a mixed state, and breast images can be classified into the following four mammary gland types according to the ratio and the distribution state of the mammary gland tissues. That is, they are four types of (1) a fatty type in which a breast region is substantially completely replaced by fat; (2) a mammary gland scattered type in which a mammary parenchyma is scattered in the breast region replaced by fat; (3) an uneven and high-density type in which the fat is mixed in the mammary parenchyma and presenting uneven density; and (4) a high-density type in which little fat is mixed in the mammary parenchyma.
FIGS. 8A and 8B show examples of signal value distribution in the mammary gland region. FIG. 8A is an example of the high-density type, and in the case of this high-density type breast, a density of the mammary gland tissues is high and an image signal value becomes the same degree, and thus, a sufficient contrast cannot be obtained, and discovery of a lesion portion is difficult. On the contrary, FIG. 8B is an example of the mammary gland scattered type, and in the case of this mammary gland scattered type breast, a high-density fat region is mixed in the mammary gland region, which causes a variation in the density of the mammary gland tissue and enhances the contrast of the mammary gland region, and thus, discovery of the lesion portion is relatively easy.
Also, FIGS. 9A and 9B show histogram shapes of the signal values corresponding to FIGS. 8A and 8B, respectively. As shown in FIG. 9A, if the mammary gland amount is large, the distribution of the signal values becomes narrow, while in the case of FIG. 9B, there is also a variation in the image signal values.
Then, application of preferable image processing according to image features of each has been carried out by image processing, giving consideration to the variation in the signal values in such a mammary gland region and analyzing the breast image in detail.
For example, such a method is known that the mammary gland amount is determined on the basis of a difference in the shape of the cumulative histogram and a preferable image processing condition is set according to the mammary gland amount (See Japanese Patent Application Laid-Open No. 2002-8009, for example).
Also, such a method is known that a mammary gland region is extracted based on the density of a pectoral muscle and the image processing condition is changed according to the mammary gland amount in the breast (See Japanese Patent Application Laid-Open No. 2002-125961, for example).
However, even if the mammary gland amount or the cumulative histogram shape is similar, the image processing condition suitable for each might be different depending on a difference in the contrast in the mammary gland.
Thus, as an image processing method using not only the mammary gland amount but also information relating to the signal value such as the contrast in the mammary gland is proposed. Specifically, the method is proposed, in which a contrast in the mammary gland is calculated by taking a difference between an average signal value of a high-density side 10% in a mammary gland structure and an average signal value of a low-density side 10% in the mammary gland structure, and the image processing condition is determined on the basis of the contrast in the mammary gland and the mammary gland amount (area ratio of the mammary gland region) (See Japanese Patent Application Laid-Open No. 2006-122445, for example).
Further, a method is known in which variance of the image signal values in the mammary gland region is calculated as the contrast in the mammary gland, and the high-density type and the others are classified by plotting the variance of the image signal values and a ratio of the mammary gland region to the breast region on a vertical axis and a horizontal axis (See Japanese Patent Application Laid-Open No. 2006-263055, for example).